A variety of electron microscopes and associated surface analyzers have evolved in recent years. A popular type is a scanning electron microscope (SEM) in which a focused electron beam is rastered over a sample surface. Secondary electrons emitted from the surface are detected in correlation with raster position. The secondary electron signals are processed electronically to provide a picture or image of topographical features of the surface. Such a microscope is described, for example, in a text "Scanning Electron Microscopy" by O. G. Wells, A Boyde, E. Lifshin and A. Rezanowich (McGraw-Hill, 1974). One common limitation of SEM is imaging the surface of insulators, because of rapid charge buildup from the incident beam of electrons. Conductive coatings or other techniques are used to alleviate this, but at the loss of surface details, time and cost of extra preparation, and loss of ability to remove surface layers during analysis.
Another method for analyzing surfaces utilizes secondary Auger electrons generated at the sample surface by a focused primary electron beam. Auger microprobes are suitable for detecting elements with atomic numbers equal to or greater than 3, and have sensitivity to only outermost atomic layers. Surface mapping of elements is accomplished by scanning with the primary electron beam. An example of a scanning Auger microprobe utilizing a cylindrical electrostatic electron analyzer is provided in U.S. Pat. No. 4,048,498.
Another approach to surface analysis is electron spectroscopy for chemical analysis (ESCA) which involves irradiating a sample surface with ultraviolet or preferably X-rays and detecting the characteristic photoelectrons emitted. The latter method is also known as X-ray photoelectron spectroscopy (XPS). The photoelectrons are filtered ("analyzed") by electrostatic or magnetic means which allow only electrons of a specified energy to pass through, similar to Auger analysis. The intensity of the resulting beam represents the concentration of a given chemical constituent of the sample surface. U.S. Pat. No. 3,766,381 describes such a system, including an electrostatic hemispherical type of analyzer. Such an analyzer also is used commonly for analysis of Auger electrons.
Elastically backscattered electrons (BSE) are also used for imaging, such as described in an article "Imaging with Backscattered Electrons in a Scanning Electron Microscope" by V.N.E. Robinson, Scanning, 15-26 (1980). These electrons are those electrons from an electron gun that are scattered by the surface with little or no energy loss. Images may be generated with special detectors as disclosed in the article. Another method is to use an energy analyzer for the BSE, as indicated in a technical reprint "How to Obtain Backscattered Electron Images" by D. P. Paul (undated), for use with Perkin-Elmer scanning Auger microprobes.
An optical microscope has generally been used to position specimens for ESCA analysis of a selected target area, or to determine area being analyzed. A simple microscope was adequate in the past, but advances in instruments have reduced the size of the analysis area, thus placing new demands on the microscope, including more magnification, shallower depth-of-field and greater stability. The microscope must be aligned to the center of the analysis area, a procedure that is inconvenient and subject to a variety of errors. The errors become more significant for smaller analysis areas, and alignment errors are not obvious during use of the system and, therefore, can lead to wrong analytical results. More magnification and shallower depth-of-field require that the objective lens for the microscope be larger in diameter and/or closer to the specimen, thereby using valuable solid angle about the specimen which could be used for other apparatus. Also, the analysis position can have a small dependence on kinetic energy, so the optical alignment can be exact only at one energy, resulting in an error at other energies. Adapting to these requirements results in a significantly more expensive optical microscope.